As noted by T. H. P. Chang, Jour. Vac. Sci. Tech., vol. 12 (1975), 1271-1275, and by others, a uniform exposure by an incident electron beam can produce a non-uniform distribution of energy deposition in the pattern area, due to backscattering of the electrons both within the directly exposed material and within any substrate serving as backing for that material. A large pattern element will receive more exposure than a small pattern element, and an isolated pattern element will receive less exposure than a pattern element that is adjacent to other pattern elements that are exposed to the electron beam. Further, a small exposed region positioned at the center of the pattern element will receive more exposure from the adjacent regions that surround that central region, than will a small region positioned adjacent to an edge of a pattern element. If a predetermined group of lines is irradiated, the absorbed energy spread surrounding each of these lines will depend upon line width and line-to-line gap width, among other things. Chang, op. cit., noted that variation of exposure dosage for line widths and gap widths less than one .mu.m is particularly troublesome here at a beam energy of 20 keV.
Previous workers have sought to compensate for the proximity effect and other effects that reduce the resolution in electron beam lithography by a number of approaches.
Greeneich observes in U.S. Pat. No. 4,264,711 that proximity effects result in substantially reduced electron beam exposure near the edges of a pattern element, as compared to net beam exposure in the interior of the pattern element. Greeneich remedies this nonuniformity within the pattern element by exposing the perimeter of a pattern element to the electron beam for a substantially greater time duration than the exposure time for an interior region within the pattern element.
Owen and Rissman, in U.S. Pat. No. 4,463,265, disclose use of a first beam irradiation that exposes the selected pattern and provision of a second electron beam irradiation that exposes all points in the complementary pattern on the surface of the material. The two beam irradiations are carried out in separate time intervals, and the beam parameters of the second irradiation are chosen to attempt to provide a uniform background of energy deposition due to electron backscattering throughout the material. The discussion in this patent is incorporated by reference herein.
Nakasuji et al., in U.S. Pat. No. 4,743,766, also proposes to provide two electron beam irradiations, in the same manner to correct for proximity effects. However, rather than using two separate exposures, Nakasuji et al. discloses use of a special e-beam tool that attenuates and de-focuses the beam rather than blanking the beam away from the irradiated pattern.
U.S. Pat. No. 4,746,587, issued to Nicholas, discloses use of the Owen and Rissman approach, using a cathode projection system and selective photoemission of electrons.
In applications of electron beam lithography to fabrication of integrated circuits, the lithography is often applied to patterning of an electron beam resist layer, attached to a substrate, through which the electron beam is to pass.
Incident electrons that pass through the resist material are scattered, and even backscattered, by atoms in the substrate material so that a significant fraction of these incident electrons return to the resist material, producing an undesired exposure of the resist layer that reduces the desired contrast of the pattern in the resist layer. The contrast of a patterned region then becomes dependent on the pattern density in that region so that proximity effect creates dimensional errors whose magnitudes depend on pattern density. Note that the proximity effect does not, as is commonly asserted, result in a loss of resolution. An example of the effect of scattering by substrate atoms is illustrated in FIG. 1 of the Owen and Rissman patent, op. cit.
Using Monte Carlo electron transport simulations, Chang, op. cit., has experimentally studied the exposure intensity distribution ("EID", a measure of electron energy deposition) of a 25 keV electron beam incident upon a silicon wafer covered with 0.6 .mu.m of PMMA resist and developed at room temperature for sixty seconds. Chang found that the EID can be closely approximated by the sum of two Gaussian distributions, namely E(r)=C.sub.1 exp [-(r/B.sub.1).sup.2 ]+C.sub.2 exp [-(r/B.sub.2).sup.2 ], where B.sub.2 /B.sub.1 &gt;&gt;1 and C.sub.2 /C.sub.1 &lt;1. The second term in the EID function E(r) represents additional exposure of the resist by the backscattered electrons, and the phenomenon associated with this is known as the proximity effect. For an incident electron beam having a beam width after focusing of the order of 0.5 .mu.m or less, the radius of the region exposed by the backscattered electrons can be of the order of 2 .mu.m at 20 keV initial beam energy. The ratio of the total energies deposited in the resist by the backscattered and toward traveling electrons is conventionally denoted by .eta..sub.e. With reference to Chang's formulation, ##EQU1## .eta..sub.e is commonly known as the "backscattered energy coefficient."
Jackel et al., in Appl. Phys. Lett., vol. 45 (1984), pp. 698-700 report experimental measurements of he, ranging from 0.70-0.78, for initial beam energies E of 20-120 keV and conclude that he is approximately independent of beam energy. Jackel et al. also find that the electron range .beta. in a resist material increases approximately as .beta..alpha.E.sup.1.7, which agrees with conclusions of Parikh and Kyser, Jour. Appl. Phys., vol. 50 (1979), pp. 1104-1111, who studied electron beam scattering in a resist material using Monte Carlo simulations.
The effect of the backscattered electrons on contrast is discussed and illustrated in the Owen and Rissman patent, op. cit., in connection with FIGS. 3A, 3B, 3C, 4A, 4B and 4C therein and is incorporated herein by reference. It is found that the heights of a sequence of peaks corresponding to desired pattern lines from electron beam lithography do not have the same maximum heights (peaks) or the same minimum heights (valleys) so that the amounts of exposure at the "centers" of the pattern lines are not uniform. The amount of exposure in the spaces between adjacent lines also is not uniform.
Several methods have been proposed to compensate for the proximity effect, including (1) compensation by dose correction, (2) compensation by pattern shape correction and (3) compensation by use of multi-level resist films. Each of these techniques incompletely compensates for the backscattered energy distribution so that some error in pattern dimensions is still present. A method is needed that more precisely compensates for the presence of, and deposition of additional beam energy due to, the back-scattered electrons within the resist material. Preferably, the method should allow irradiation of the exposed areas of the resist material with a single pass, rastered or vector-scanned electron beam.